This invention relates to a multiplexing/demultiplexing method, in particular for conveying a plurality of binary digital signals within a single higher rate binary digital signal and which does not require an alignment process at the receiving end in order to decompose the combined signal into its constituant parts and convey them to their correct destinations.
Known multiplexing/demultiplexing methods use a process often referred to as "alignment" in order to unambiguously identify the constituent parts of the combined signal. For example some multiplexing methods use bit or byte interleaving of the lower rate signals, and include in the transmitted signals some extra bits, e.g. those of a frame alignment word, parity coding, or carter coding. Other methods use block coding with parallel inputs and a serial output.
At the receiving end where demultiplexing is to take place the received signal must be disinterleaved or decoded and the start and finish of each frame or code block must be known. This is often referred to as "alignment", a process which comprises searching in either serial or parallel form for correct alignment by counting the number of correct false and consecutive alignment positions as an aid to stopping or starting the search procedure, or otherwise deducing the start and finish of each frame or block from some property added to them by the multiplexing method. Known examples are the frame structures defined in ITU recommendations G.741 and G.751.
Another known method, which does not of necessity require that the transmitted rate is greater than the sum of the individual rates of the signals being multiplexed together, uses bit or byte interleaving, but depends upon at least one of the individual signals having a known property of its bit sequence which is different from the others, e.g. a frame word, so that it can be identified. This may be regarded as a special case of the method described previously in which extra bits are incorporated into one of the individual signals before multipexing. It still requires a process of alignment during demultiplexing.